Microsoft has given web developers a last chance to test their sites against an early build of Internet Explorer 9 before the browser is released in beta form to a more general audience.

Internet Explorer 9 (IE9) Platform Preview 4 was released on Wednesday evening, six weeks after the third platform preview. The beta is roughly scheduled for September, while there is no indication yet as to when the final version of the browser will be released.

According to Mark Quirk, Microsoft UK's web product manager, there have been 2.4 million downloads of the first three IE9 platform previews, a result with which Microsoft is "quite pleased".

"Our desire ultimately is that web developers test their sites against Platform Preview 4," Quirk told ZDNet UK on Wednesday. "If something is wrong with our browser, let us know. We would hope to fix it before the beta."

Apart from getting much better at handling HTML 5 — an advance that became particularly apparent in Platform Preview 3 — IE9 will also have significant performance improvements over IE8, which came out in early 2009. Much of this performance boost comes from making more use of intelligent hardware, where available, such as the graphics processing unit (GPU). According to Microsoft's tests, SunSpider benchmarking for IE9 Platform Preview 4 provides a score of 326ms. In contrast (with lower scores indicating faster JavaScript performance), Platform Preview 3 scored 347ms and IE8 scored a sluggish 3,746ms.

Since the third platform preview, IE9's Acid3 score — a test of the browser's compliance with web standards — went up from 83/100 to 95/100. Quirk said he did not expect that score to change before the beta release.

Quirk explained that IE9 does not score 100/100 on the Acid3 test — as rivals such as Chrome do — because of its avoidance of a few specifications. One is the Synchronized Multimedia Integration Language (SMIL) specification for multimedia specifications — according to Quirk, the SVG working group behind SMIL has proposed changes to the specification, which has dissuaded Microsoft from implementing the "not that widely used" spec.

Microsoft is also avoiding the use of scalable vector graphics (SVG) fonts in favour of using CSS3 and Web Open Font Format (WOFF) fonts instead. "This is generally recognised as the right way of doing fonts," Quirk said. "The typography community would rather the implementation goes down the route of WOFF and CSS3 fonts, so we believe that we're doing the right thing for the broadest audience."

Quirk also revealed that Microsoft's feedback after IE9 testing had led to a change in the EcmaScript 5th edition specification, most commonly used in the form of JavaScript.

"When we did Platform Preview 3, we implemented the EcmaScript 5 specification," he said. "In doing so and implementing it as the specification said, we found a lot of websites broke. So we escalated that to Ecma, and there's been an update to the EcmaScript standard based on what we saw. We were one of the first to implement it, and the standard was wrong."

David Meyer is a freelance technology journalist. He fell into journalism when he realised his musical career wouldn't be paying many bills. His early journalistic career was spent in general news, working behind the scenes for BBC radio and on-air as a newsreader for independent stations. David's main focus is on communications, of both...
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